It is known that isobutylene and other isoalkenes, or iso-olefins, produced by hydrocarbon cracking may be reacted with methanol and other C.sub.1 -C.sub.4 lower aliphatic alcohols, or alkanol, over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) or the like. Generally, it is known that asymmetrical ethers having the formula (CH.sub.3)3C--O--R, where R is a C.sub.1 -C.sub.4 alkyl radical, are particularly useful as octane improvers for liquid fuels, especially gasoline.
MTBE, ethyl t-butyl ether (ETBE), tert-amyl methyl ether (TAME) and isopropyl t-butyl ether (IPTBE) are known to be high octane ethers. The article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The octane blending number of MTBE when 10% is added to a base fuel (R+O=91) is about 120. For a fuel with a low motor rating (M+0=83) octane, the blending value of MTBE at the 10% level is about 103. On the other hand, for an (R+0) of 95 octane fuel, the blending value of 10% MTBE is about 114.
The liquid phase reaction of methanol with isobutylene and isoamylenes at moderate conditions with a resin catalyst is known technology, as provided by R. W. Reynolds, et al., The Oil and Gas Journal, Jun. 16, 1975, and S. Pecci and T. Floris, Hydrocarbon Processing, December 1977. An article entitled "MTBE and TAME--A Good Octane Boosting Combo," by J. D. Chase, et al., The Oil and Gas Journal, Apr. 9, 1979, pages 149-152, discusses the technology. Preferred catalysts are polymeric sulfonic acid exchange resin such as Amberlyst 15 and zeolites such as zeolite Beta and ZSM-5. The acid resin catalysts are effective catalysts at temperatures below 90.degree. C. At higher temperatures the resin catalyst is unstable. Typically, with acid resin catalyst the etherification reaction is carried out in liquid phase. However, mixed phase etherification is known, particularly where the catalyst is contained as a fixed bed in a fractionator which serves to both separate the reaction products and operate as a vessel to contain the catalyst under etherification conditions.
Typical hydrocarbon feedstock materials for etherification reactions include olefinic streams, such as cracking process light gas containing butene isomers in mixture with substantial amounts of paraffins including n-butane and isobutane The C.sub.4 components usually contain a major amount of unsaturated compounds, such as 10-40% isobutylene, 20-55% linear butenes, and small amounts of butadiene. Also, C.sub.4 +heavier olefinic hydrocarbon streams may be used, particularly mixtures of isobutylene and isoamylene and C.sub.5 + streams containing isoamylene. To augment the sources of feedstock for C.sub.4 -C.sub.5 isoolefins for conversion to MTBE and TAME it is desirable to employ light olefins-rich fractions from olefinic gasoline, typically the light naphtha fraction from fluid catalytic cracking (FCC) processes.
In M. N. Harandi U.S. Pat. No. 4,886,925, incorporated herein by reference, an integrated process is disclosed for the conversion of C.sub.2 + normal olefins into higher isoolefins with etherification to provide methyl tertiary alkyl ethers and high octane gasoline. The process combines olefins interconversion with etherification and conversion of unreacted methanol and olefins in contact with acidic, shape selective metallosilicate zeolite catalyst.
In the process for catalytic conversion of oxygenate and/or olefins to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of either gasoline, distillate, lube range products or aromatics. Light olefins can be oligomerized to high molecular weight distillate range olefins over ZSM-5. Olefin molecular weight growth through a sequence of oligomerization and cracking reactions is thermodynamically forced at relatively high pressures of about 5600 kPa (800 psia) and relatively low temperatures of about 260.degree. C. (500.degree. F.). At much lower pressure and higher temperature, thermodynamics restrict the olefin distribution to low molecular weight. This is the basis for the olefin interconversion process, i.e., to operate under conditions where lower olefins can be converted to an equilibrium distribution of olefins with iso-butenes and iso-pentenes maximized. The olefin interconversion process can use fixed bed, moving bed or fluid bed reactors containing zeolite type catalysts such as ZSM-5. Operating conditions encompass temperatures between 200 and 400.degree. C. and low pressures, generally between 100 and 1500 kPa. The olefin interconversion process provides a unique method to optimize the concentration of isoolefins in a mixture of light olefins.
Accordingly, it is an object of the present invention to provide a process for the production of high octane value alkyl tertiary alkyl ethers.
Another object of the invention is to provide high octane value ethers using a light gasoline fraction such as FCC light naphtha as feedstock by conversion to isoolefin-rich feedstock.
Another object of the invention is to provide a process for the production of isoolefins for MTBE and TAME production by olefin interconversion of C.sub.5 -C.sub.7 olefinic hydrocarbons.